1. Field of the Invention
The present invention generally relates to a vane pump and, more particularly, to a vacuum vane pump for use on aircraft to generate a vacuum condition necessary for the operation of aircraft instrumentation.
2. Description of the Related Art
Sensitive analog guidance instrumentation and indicator displays such as gauges used on small aircraft and principally relied upon by aircraft pilots in the operation of such aircraft require a suitable operating environment to properly function. Such instrumentation are highly susceptible to variations in their operating environment and, in particular, to changes in barometric or cabin pressure which will affect, among other things, altimeter measurements and, hence, the gauge displays of such measurements. For these reasons, it is imperative that such guidance equipment and indicators operate in as close to a vacuum condition as practicable so that they provide a true indication of aircraft operating parameters.
Vane pumps are commonly used to generate the necessary vacuum for small aircraft instrumentation. With reference to FIG. 1, known vane pumps 5 typically consist of a rotor 10 secured for rotation with a motor-driven shaft 12 by a key 13. The rotor 10 has a peripheral surface 14 and is surrounded by a casing or housing 30 having an inward facing surface 32. Rotor 10 is mounted eccentrically in housing 30 so that its peripheral surface 14 and the confronting inner surface 32 of housing 30 define a crescent-shaped chamber 34 therebetween. The rotor also has a plurality of radially-extending slots 16 defined and extending between an inner end 17 and an outer end 19 coterminous with peripheral surface 14. A plurality of fingers or vanes 20 (five of which are shown by way of illustrative example) are slidably engageable in slots 16. Each vane 20 has an inner end 21 disposed proximate end 17 of its corresponding slot, and an outer tip 22 which as the vane 20 is radially-outwardly displaced along its slot 16 by the centrifugal force exerted on the vanes through rotation of rotor 10, is driven beyond outer end 19 and into pressed abutment against the internal surface 32 of housing 30. As rotation of the rotor causes vane tips 22 to pressingly advance circumferentially about the inner surface 32 of housing 30, the air or gas in the crescent-shaped chamber 34 is alternately compressed and expanded to create a vacuum.
In order to extend the resulting vacuum to the appropriate instruments in, for example, an aircraft cockpit, porting of vacuum chamber 34 is effected, as is known in the art, for example in the manner shown in U.S. Pat. No. 5,100,308 which also describes multiple stages and arrangements for changing the eccentricity of the rotor in the housing to enable regulation of the amount of vacuum or positive pressure generated by the vane pump. The slots and vanes need not be oriented in a strictly radial manner but may, instead, be skewed as for example disclosed in FIGS. 6 and 7 of U.S. Pat. No. 6,086,332.
The vanes 20 in a conventional vane pump of the type heretofore described and depicted in FIG. 1 are typically formed of carbon and the housing 30 is typically constructed of aluminum. During pump operation the vane tips 22 incur steady wear from friction as they press against and slidingly contact the inner housing surface 32 during rotation of rotor 10. Over time, the gradual wearing of the tips 22 causes shortening of the vanes relative to their respective slots 16, thus requiring that the vanes be displaced or travel farther radially outward along the slots; as a result, for each one the distance between inner slot end 17 and inner vane end 21 increases as the outer tip 22 continues to frictionally engage surface 32. Put another way, the length of each vane 20 that remains within its respective slot 16 as the vane tip 22 is centrifugally pressed against confronting housing wall surface 22 steadily decreases as the pump 5 is operated. Eventually, one or more of the vanes decrease in length to a point that its portion that extends out of its respective slot 16 approaches or exceeds in length the portion that remains within the slot. This condition commonly results in a sudden and catastrophic breaking or dislodging of one or more of the vanes from their respective slots in or about the widest part of chamber 34, causing an abrupt loss of vacuum. Often one broken vane initiates a catastrophic destruction of additional ones of vanes and, therefore, a complete loss of vacuum. Unexpected reduction or total loss of vacuum adversely affects instrumentation readings and can lead to erroneous decisions in controlling the aircraft. Even with a vacuum loss warning system, the information displayed to a pilot on the gauges is unreliable at best.
The long-accepted solution, according to the prior art, is regular scheduled maintenance of vane pumps through periodic disassembly to detect the extent of vane wear, a time consuming and costly procedure that, without early replacement of pumps typically capable of safely operating for additional extended periods of time, is an unreliable predictor of imminent pump failure.